Split BrowserThread into public API and private implementation, step 1.

chrome/browser/io_thread.h

Index: chrome/browser/io_thread.h
diff --git a/chrome/browser/io_thread.h b/chrome/browser/io_thread.h
index b4fd0a8c63e2ae220ae211a3ce9ecadf0d1d21cf..1918e6e1e1e25d757c39fd6aa5c6674c4e3dec76 100644
--- a/chrome/browser/io_thread.h
+++ b/chrome/browser/io_thread.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <string>
+
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
@@ -48,7 +49,7 @@ class URLRequestContextGetter;
 class URLSecurityManager;
 }  // namespace net
 
-class IOThread : public BrowserProcessSubThread {
+class IOThread : public content::BrowserProcessSubThread {
  public:
   struct Globals {
     Globals();